1. Field of the Invention
The present invention relates to a pile driver, and in particular relates to a structure of a leader revolver that is a leader support device including a mechanism for rotating a rotary leader provided in a pile driver.
2. Description of the Related Art
A pile driver including a rotary leader that is supported rotatably about an axis is known for use in auger-hammer combined operations and sheet/multiaxial method operations. As a structure for rotatably supporting the leader and also rotating the leader, the leader is divided into an upper leader and a bottom leader and a hydraulic actuator is provided between the upper and the bottom leader to rotate the upper leader. Known systems for rotatably supporting the upper leader by the bottom leader include a system in which the upper leader and the bottom leader are simply connected together via a ball bearing, and a system in which a casing flange (fixed flange) of a hydraulic motor including a speed reducer is connected to an upper part of the bottom leader and a rotary case flange (movable flange) of the hydraulic motor is connected to a lower part of the upper leader (for example, see Japanese unexamined Patent Publication (Kokai) No. Sho 58-195624). In addition, there is also a known system in which a subleader is mounted on a front part of a base machine and a main leader is rotatably supported by leader holders provided at the top and bottom of the subleader, with a leader rotation device being incorporated in the bottom leader holder (for example, see Japanese Unexamined Utility Model Registration Publication (Kokai) No. Sho 61-89040).
In the leader support system that is eccentric to the center of the leader, when a large bending moment is applied to the leader, there is a danger that an excessive load acts on the ball bearing or the flange of the hydraulic motor and impairs its function or causes damage. Moreover, in the system where the upper and lower flanges of the hydraulic motor including the speed reducer are connected respectively to the upper and bottom leaders, it is extremely difficult, with strength of a typical hydraulic motor, to tolerate a large bending moment applied to the upper leader. Therefore, a general-purpose hydraulic motor cannot be used, and a special, expensive hydraulic motor is necessary. For example, a vane hydraulic motor is used as a means for rotating the upper leader while firmly holding the upper leader. However, a large vane hydraulic motor has a problem that much labor is required for polishing to ensure accuracy. Hence increase in cost cannot be avoided in this case, too.